A feedthrough generally allows an electrical signal or electrical power to pass through a wall or similar structure. Feedthroughs may be used to pass one or more conductors from an enclosure of an electronic device to an ambient environment, or to pass one or more conductors to another compartment of the electronic device, while maintaining electrical isolation from the enclosure, other feedthroughs, and other conductive connections or circuitry.
While feedthroughs are widely used, a relatively smaller set of feedthroughs are used to ensure that ignitions that may occur within an enclosure or a specific compartment of the device are not propagated through the feedthrough and ignite either the surrounding atmosphere or atmosphere in another compartment. An ignition inside a sufficiently robust housing will also induce appreciable pressure, both due to the initial explosion pressure, and due to secondary ignitions or pressure waves due to pressure piling. Accordingly, the feedthrough's construction must also be able to withstand the pressure of such an explosion, in order to contain the ignition source. Further, the feedthrough joint may permit the products of combustion, and the resulting expansion of gases, to be relieved through the joint but may not permit the flame front to ignite the atmosphere either in another compartment of the device or external to the housing. This is generally accomplished with a suitable flame-quenching pathway. This is particularly important in environments where an electrical device may be operating in an atmosphere that is flammable or even explosive. In such cases, the feedthrough must be designed to comply with applicable standards to ensure that if the circuitry within the enclosure could generate a spark with sufficient energy to ignite the atmosphere, or if the circuitry has components with high enough surface temperatures that could generate ignition, that such flame front cannot pass through the feedthrough, and the feedthrough can withstand the resulting explosion pressure. Terminating the feedthrough for permanent wiring or circuit connections, whether during construction of the device or upon installation of the device, may be realized with a terminal block or wiring. Terminal blocks and wiring used in explosive atmospheres may make use of non-sparking, non-arcing construction such that the spacing of the conductors, and the insulative material of the terminal block itself ensures adequate isolation of the electrical connections from other conductive mediums. This isolation and securing of electrical potentials one from another ensures that no ignition of the flammable atmosphere can occur as no potential arc-points are created. This protection concept is widely referred to in a number of ways, including non-arcing, non-sparking, type e (increased safety), or nonincendive equipment.
One common way in which flame-proof or explosion-proof feedthroughs are provided is by passing one or more wires through a cemented joint. In order to create the cemented joint, a cement or suitable mixture, such as epoxy or a similar resin system, is poured or otherwise formed between the wires and a sidewall of the feedthrough. The sidewall of the feedthrough may then be coupled to a suitable aperture in an enclosure. In some instances, the cement or mixture may extend directly from the wire(s) to the enclosure wall. When the cement or epoxy cures or otherwise hardens, the result is a robust construct that is able to resist the pressure of an internal explosion, and provides a seal from flame propagation from one side of the joint to the other.
One limitation of a cemented or epoxied explosion-proof joint is that the quality and/or consistency of the cemented joint is determined by the mixture, the mixing process, and/or the presence of air bubbles in the cement/epoxy compound during joint formation. Further still, once the cemented joint has hardened or cured, it is still occasionally possible for the joint itself to crack thereby providing a potential path for a flame through the joint. Another limitation of cemented flame-proof joints is that they are relatively labor intensive to create.
Providing an easy to implement, low cost flame-proof/increased safety/non-arcing feedthrough would significantly benefit the industries that provide electronic devices for such flammable environments.